Re: [PATCH 00/25] Change time_t and clock_t to 64 bit

Thomas Gleixner
2014-05-14T12:21:41Z

On Wed, 14 May 2014, Arnd Bergmann wrote:
> On Tuesday 13 May 2014 22:35:08 Geert Uytterhoeven wrote:
> >
> > On Tue, May 13, 2014 at 9:32 PM, Arnd Bergmann <arnd@arndb.de> wrote:
> > > I think we have three categories:
> >
> > Thanks for the list!
> >
> > > a) interfaces that uses relative time_t/timespec/timeval:
> > > b) interfaces that don't make sense for times in the past:
> >
> > > c) interfaces that require absolute times:
> > > - stat/lstat/fstatat/
> > > - utime/utimes/futimesat
> > >
> > > These absolutely have to use something better than time_t
> > > both in user space and in the kernel so we can deal with
> > > old files. A lot of file systems need to be fixed as well so
> > > we can actually store the times, regardless of whether we
> > > are running a 32 or 64 bit kernel.
> >
> > So these are the ones we have to worry about.
> > It looks like they all involve I/O? Apart from the case of using block data
> > from the buffer cache, the 64-bit operations should disappear in the
> > actual I/O noise, right?
>
> Right. Also there have been proposals for a better 'stat' replacement
> for years, which would solve half of the interface problem for the
> file system interfaces.
>
> However, we also need to find a solution for category b), I only put
> them into a different category above because we can treat them
> differently in the kernel. For instance, we could use ktime_t for
> the kernel code in category b) and a new struct timespec64 for
> the times in struct inode.
> On the user interface side, using timespec64 would be a reasonable
> choice for both categories, because we already have two implementations
> of all those syscalls in order to handle 32-on-64 compat tasks,
> and we could use the same set of syscall implementations for time64-on-32.

So in the 32-on-64 case we'll have two compat variants:

SYSCALL_DEFINE6(futex, u32 __user *, uaddr, int, op, u32, val,
struct timespec __user *, utime, u32 __user *, uaddr2,
u32, val3)

COMPAT_SYSCALL_DEFINE6(futex, u32 __user *, uaddr, int, op, u32, val,
struct compat_timespec __user *, utime, u32 __user *, uaddr2,
u32, val3)

COMPAT_SYSCALL_DEFINE6(futex64, u32 __user *, uaddr, int, op, u32, val,
struct timespec64 __user *, utime, u32 __user *, uaddr2,
u32, val3)

The native 64bit futex64 syscall is mapped to futex.

And for a 32bit kernel you have two

SYSCALL_DEFINE6(futex, u32 __user *, uaddr, int, op, u32, val,
struct timespec __user *, utime, u32 __user *, uaddr2,
u32, val3)

SYSCALL_DEFINE6(futex64, u32 __user *, uaddr, int, op, u32, val,
struct timespec64 __user *, utime, u32 __user *, uaddr2,
u32, val3)

Fine with me, but we really need to discuss the timespec64 with user
space folks.

I'm curious, whether quite some code, like high frequency timestamps
wouldn't be better of with a strict 64 bit nanosecond granular time
represenation.

I often enough cursed timespec for clock_nanosleep on an absolute
timeline. I need to go through all that normalizing stuff instead of
just doing next_event += 500000;

Thanks,

tglx